The present invention relates to a synthetic quartz glass for an optical member, a process for producing the same and a method of using the same, particularly to a synthetic quartz glass for an optical member, which shows high transmittance of vacuum ultraviolet light having a wavelength of 165 nm or shorter, a process for producing the same and a method of using the same.
In recent years, along with high integration of LSI, a technique for fine patterning with a narrower line width has been required in the photolithographic technology, and shortening of the wavelength of the light for exposure is being advanced. For example, the light source for a lithographic stepper has been advanced from conventional g-line (wavelength: 436 nm) or i-line (wavelength: 365 nm) to adopt a KrF excimer laser (wavelength: 248 nm) or an ArF excimer laser (wavelength: 193 nm). Further, as a light source for next generation where a finer patterning technique is required, a fluorine laser (wavelength: 157 nm) is mentioned as a prospective candidate.
In an optical system using a KrF excimer laser or an ArF excimer laser as a light source, a synthetic quartz glass is employed, since it is transparent over a wide wavelength range from a near infrared region to a vacuum ultraviolet region, and it is excellent in various properties such that the thermal expansion coefficient is very small so that it is excellent in dimensional stability, and it has high purity. However, a synthetic quartz glass having a large OH group content, which is used for a KrF excimer laser or an ArF excimer laser, has a low transmittance in a region of wavelengths of not longer than 165 nm, and as such, it is not useful for a fluorine laser which is expected to be a light source of next generation.
Further, with a synthetic quartz glass having the OH group content reduced in order to improve the transmittance in a region of wavelengths of not longer than 165 nm, the transmittance sharply decreases as the wavelength of the transmitted light becomes shorter than about 170 nm.
Accordingly, in a case where a synthetic quartz glass is to be used as an optical member in an optical system employing a fluorine laser as a light source, it will be an important subject to improve the transmittance.
Further, the optical system of an optical apparatus to be used for photolithography, is constituted by a combination of many optical members such as lenses and prisms. Accordingly, an improvement in transmittance of each individual optical member will bring about a large improvement in transmittance when integrated as an entire optical system.
However, heretofore, there has been no proposal for a method to improve the transmittance in a region of wavelengths of not longer than 165 nm which is the wavelength region of the fluorine laser or to produce a synthetic quartz glass for an optical member having high transmittance efficiently and conveniently.
The present invention has an object to provide a process whereby a synthetic quartz glass for an optical member having the transmittance in a region of wavelengths of not longer than 165 nm improved, can be produced efficiently and conveniently.
The present invention further provides a synthetic quartz glass for an optical member having a high transmittance at a wavelength of 157 nm and a method of using it.
The present invention provides a process for producing a synthetic quartz glass for an optical member, which comprises a step of irradiating a synthetic quartz glass having an OH group content of 50 ppm (weight ppm, the same applies hereinafter) or lower with vacuum ultraviolet light having a wavelength of 180 nm or shorter to improve the transmittance in a region of wavelengths of not longer than 165 nm.
Further, the present invention provides a synthetic quartz glass for an optical member having an absorption coefficient of at most 0.70 cmxe2x88x921 at a wavelength of 157 nm.
Still further, the present invention provides a method of using a synthetic quartz glass for an optical member, which is a method of using a synthetic quartz glass for an optical member having an OH group content of 50 ppm or lower, and which comprises a step of irradiating it with vacuum ultraviolet light having a wavelength of 180 nm or shorter before using it as an optical member.